The phagocytic indices of hemolymph cells from Crassostrea virginica and Mercenaria mercenaria exposed to bacteria have been ascertained. These data indicate that it is the granulocyte which is the most important from the standpoint of phagocytosis. Also, it has been determined that phagocytosis is temperature dependent, being essentially nonexistent at 4 degrees C. Certain lysosomal enzymes have been detected and quantified in both the serum and cells of the hemolymph of C. virginica, M. mercenaria, and Mya arenaria. Furthermore, it has been ascertained that the serum enzymes are released from cells primarily during phagocytosis. The encapsulation process in molluscs to nematodes haq been examined and it has been hypothesized that the nondestruction of certain nematodes, such as Angiostrongylus cantonesis, is due to the nontriggering of release of hydrolytic enzymes from cells forming the encapsulation complex.